The automatic bridge defense system, as its name suggests, is a computer-controlled defensive weapon found on the bridge of Federation starships around 2269. It fired a phaser-like beam.
When a malevolent lifeform took control of the USS Enterprise in 2269, it tried to use the system to force the crew's obedience. (TAS: "Beyond the Farthest Star")
See also
Automatic bridge defense system deployed
The system fires on Spock